Gastab

A histamine congener, it competitively inhibits histamine binding to histamine H2 receptors. Gastab has a range of pharmacological actions. It inhibits gastric acid secretion, as well as pepsin and gastrins output. It also blocks the activity of cytochrome P-450 which might explain proposals for use in neoadjuvant therapy. [PubChem]

Gastab Tablets USP are indicated in:

Gastab is in a group of drugs called histamine receptor antagonists. Gastab works by decreasing the amount of acid your stomach produces.

Gastab is used to treat and prevent certain types of ulcer, and to treat conditions that cause the stomach to produce too much acid. Gastab is also used to treat gastroesophageal reflux disease (GERD), when stomach acid backs up into the esophagus and causes heartburn.

Gastab may also be used for other purposes not listed in this medication guide.

Duodenal Ulcer

Clinical studies have indicated that suppression of nocturnal acid is the most important factor in duodenal ulcer healing. This is supported by recent clinical trials. Therefore, there is no apparent rationale, except for familiarity with use, for treating with anything other than a once-daily at bedtime oral dosage regimen.

In a U.S. oral dose-ranging study of 400 mg at bedtime, 800 mg at bedtime and 1600 mg at bedtime, a continuous dose response relationship for ulcer healing was demonstrated.

However, 800 mg at bedtime is the dose of choice for most patients, as it provides a high healing rate (the difference between 800 mg at bedtime and 1600 mg at bedtime being small), maximal pain relief, a decreased potential for drug interactions and maximal patient convenience. Patients unhealed at four weeks, or those with persistent symptoms, have been shown to benefit from two to four weeks of continued therapy.

It has been shown that patients who both have an endoscopically demonstrated ulcer larger than 1 cm and are also heavy smokers (i.e., smoke one pack of cigarettes or more per day) are more difficult to heal. There is some evidence which suggests that more rapid healing can be achieved in this subpopulation with Gastab 1600 mg at bedtime. While early pain relief with either 800 mg at bedtime or 1600 mg at bedtime is equivalent in all patients, 1600 mg at bedtime provides an appropriate alternative when it is important to ensure healing within four weeks for this subpopulation. Alternatively, approximately 94% of all patients will also heal in eight weeks with Gastab 800 mg at bedtime.

Other Gastab oral regimens in the U.S. which have been shown to be effective are: 300 mg four times daily, with meals and at bedtime, the original regimen with which U.S. physicians have the most experience, and 400 mg twice daily, in the morning and at bedtime.

Concomitant antacids should be given as needed for relief of pain. However, simultaneous administration of oral Gastab and antacids is not recommended, since antacids have been reported to interfere with the absorption of Gastab.

While healing with Gastab often occurs during the first week or two, treatment should be continued for 4 to 6 weeks unless healing has been demonstrated by endoscopic examination.

Maintenance Therapy for Duodenal Ulcer

In those patients requiring maintenance therapy, the recommended adult oral dose is 400 mg at bedtime.

Active Benign Gastric Ulcer

The recommended adult oral dosage for short-term treatment of active benign gastric ulcer is 800 mg at bedtime, or 300 mg four times a day with meals and at bedtime. Controlled clinical studies were limited to six weeks of treatment. 800 mg at bedtime is the preferred regimen for most patients based upon convenience and reduced potential for drug interactions. Symptomatic response to Gastab does not preclude the presence of a gastric malignancy. It is important to follow gastric ulcer patients to assure rapid progress to complete healing.

Erosive Gastroesophageal Reflux Disease (GERD)

The recommended adult oral dosage for the treatment of erosive esophagitis that has been diagnosed by endoscopy is 1600 mg daily in divided doses (800 mg twice daily or 400 mg four times daily) for 12 weeks. The use of Gastab beyond 12 weeks has not been established.

Pathological Hypersecretory Conditions (such as Zollinger-Ellison Syndrome)

Recommended adult oral

Dosage: 300 mg four times a day with meals at bedtime. In some patients it may be necessary to administer higher doses more frequently. Doses should be adjusted to individual patient needs, but should not usually exceed 2400 mg per day and should continue as long as clinically needed.

Dosage Adjustments for Patients with Impaired Renal Function

Patients with severely impaired renal function have been treated with Gastab. However, such dosage has been very limited. On the basis of this experience the recommended dosage is 300 mg every 12 hours orally. Should the patient’s condition require, the frequency of dosing may be increased to every 8 hours or even further with caution. In severe renal failure, accumulation may occur and the lower frequency of dosing comparable with an adequate patient response should be used. When liver impairment is also present, further reductions in dosage may be necessary. Hemodialysis reduces the level of circulating Gastab. Ideally, the dosage schedule should be adjusted so that the timing of a scheduled dose coincides with the end of hemodialysis.

See also:
What is the most important information I should know about Gastab?

Use this medication exactly as directed on the label, or as your doctor has prescribed it for you. Do not use more of the medication than recommended. Do not use the medication for longer than recommended.

Avoid taking antacids unless your doctor recommends them for heartburn pain. Follow your doctor's advice about the type of antacid to use and when to use it. You may not be able to take the antacid at the same time you take your dose of Gastab.

Taking Gastab may make you more susceptible to virus that can cause pneumonia. This has occurred most often in elderly people and in those with diabetes, a weak immune system, or chronic lung disease. Before using Gastab, tell your doctor if you have any of these conditions.

There are many other drugs that can interact with Gastab. Tell your doctor about all the prescription and over-the-counter medications you use. This includes vitamins, minerals, herbal products, and drugs prescribed by other doctors. Do not start using a new medication without telling your doctor.

Heartburn can be confused with early symptoms of heart attack. Seek emergency medical attention if you have chest pain or heavy feeling, dizziness, pain spreading to the arm or shoulder, sweating, nausea or vomiting, and a general ill feeling.

Use Gastab suspension as directed by your doctor. Check the label on the medicine for exact dosing instructions.

• Take Gastab suspension by mouth with or without food.
• Shake well before each use.
• Take Gastab suspension with a full glass of water (8oz/240 mL).
• Use a measuring device marked for medicine dosing. Ask your pharmacist for help if you are unsure of how to measure this dose.
• If you also take cephalosporin (eg, cephalexin), itraconazole, or ketoconazole, take it at least 2 hours before taking Gastab suspension. Check with your doctor if you have questions.
• If you miss a dose of Gastab suspension and you are taking it regularly, use it as soon as possible. If it is almost time for your next dose, skip the missed dose and go back to your regular dosing schedule. Do not use 2 doses at once.

Ask your health care provider any questions you may have about how to use Gastab suspension.

Gastab is used to treat ulcers of the stomach and intestines and prevent them from coming back after they have healed. This medication is also used to treat certain stomach and throat (esophagus) problems caused by too much stomach acid (e.g., Zollinger-Ellison syndrome, erosive esophagitis) or a backward flow of stomach acid into the esophagus (acid reflux disease/GERD). Decreasing extra stomach acid can help relieve symptoms such as stomach pain, heartburn, difficulty swallowing, persistent cough, and trouble sleeping. It can also prevent serious acid damage to your digestive system (e.g., ulcers, cancer of the esophagus).

Gastab belongs to a class of drugs commonly called H2 blockers. It works by reducing the amount of acid in your stomach.

This medication is also available without a prescription. It is used to treat occasional heartburn caused by too much acid in the stomach (also called acid indigestion or sour stomach). It is also used to prevent heartburn and acid indigestion caused by certain foods and beverages. If you are taking this medication for self-treatment, it is important to read the manufacturer's package instructions carefully so you know when to consult your doctor or pharmacist.

How to use Gastab

Take this medication by mouth with or without food as directed by your doctor.

The dosage and length of treatment are based on your medical condition and response to therapy. Follow your doctor's instructions carefully. If you are also taking antacids to relieve stomach pain as recommended by your doctor, separate them from this medication by at least 1 hour.

Take this medication regularly as prescribed in order to get the most benefit from it. To help you remember, take it at the same time(s) each day. Do not increase your dose or take it more often than directed. Continue to take this medication for the prescribed length of treatment even if you are feeling better. Stopping treatment too early may delay the healing process.

If you are using nonprescription Gastab for self-treatment of acid indigestion or heartburn, take 1 tablet by mouth with a glass of water as needed. To prevent heartburn, take 1 tablet by mouth with a glass of water right before or up to 30 minutes before eating food or drinking beverages that cause heartburn. Do not take more than 2 tablets in 24 hours unless directed by your doctor. Do not take for more than 14 days in a row without talking with your doctor.

Inform your doctor if your symptoms do not improve or if they worsen.

See also:
What other drugs will affect Gastab?

Acalabrutinib: Histamine H2 Receptor Antagonists may decrease the serum concentration of Acalabrutinib. Management: To minimize the potential for a significant interaction, separate administration of these agents by giving acalabrutinib 2 hours before ingestion of a histamine-2 receptor antagonist. Consider therapy modification

Alfentanil: Gastab may increase the serum concentration of Alfentanil. Monitor therapy

Amiodarone: Gastab may increase the serum concentration of Amiodarone. Management: Consider alternatives to Gastab. If this combination cannot be avoided, monitor for increased amiodarone concentrations/effects with Gastab initiation/dose increase or decreased concentrations/effects with Gastab discontinuation/dose decrease. Consider therapy modification

ARIPiprazole: CYP3A4 Inhibitors (Weak) may increase the serum concentration of ARIPiprazole. Management: Monitor for increased aripiprazole pharmacologic effects. Aripiprazole dose adjustments may or may not be required based on concomitant therapy and/or indication. Consult full interaction monograph for specific recommendations. Monitor therapy

ARIPiprazole: CYP2D6 Inhibitors (Weak) may increase the serum concentration of ARIPiprazole. Management: Monitor for increased aripiprazole pharmacologic effects. Aripiprazole dose adjustments may or may not be required based on concomitant therapy and/or indication. Consult full interaction monograph for specific recommendations. Monitor therapy

Atazanavir: Histamine H2 Receptor Antagonists may decrease the serum concentration of Atazanavir. Management: Specific dose limitations and administration guidelines exist; consult full interaction monograph or atazanavir prescribing information. Consider therapy modification

AtorvaSTATin: May enhance the adverse/toxic effect of Gastab. Specifically, there is a theoretical potential for enhanced effects on reducing endogenous steroid activity. Monitor therapy

Azelastine (Systemic): Gastab may increase the serum concentration of Azelastine (Systemic). Monitor therapy

Bosutinib: Histamine H2 Receptor Antagonists may decrease the serum concentration of Bosutinib. Management: Administer histamine H2 receptor antagonists more than 2 hours before or after bosutinib. Consider therapy modification

Bromazepam: Gastab may increase the serum concentration of Bromazepam. Monitor therapy

Calcium Channel Blockers: Gastab may increase the serum concentration of Calcium Channel Blockers. Management: Consider alternatives to Gastab. If no suitable alternative exists, monitor for increased effects of calcium channel blockers following Gastab initiation/dose increase, and decreased effects following Gastab discontinuation/dose decrease. Exceptions: AmLODIPine; Clevidipine; NiCARdipine. Consider therapy modification

CarBAMazepine: Gastab may increase the serum concentration of CarBAMazepine. The serum carbamazepine concentration might return to normal within one week of starting Gastab. Monitor therapy

Carmustine: Gastab may enhance the myelosuppressive effect of Carmustine. Management: Consider alternatives to Gastab in patients receiving carmustine. If the combination cannot be avoided, monitor for enhanced carmustine myelotoxicity. Consider therapy modification

Carvedilol: Gastab may increase the serum concentration of Carvedilol. Monitor therapy

Cefditoren: Histamine H2 Receptor Antagonists may decrease the serum concentration of Cefditoren. Management: Concomitant use of cefditoren with H2-antagonists and antacids is not recommended. Consider alternative methods to control acid reflux (eg, diet modification) or alternative antimicrobial therapy if use of H2-antagonists can not be avoided. Consider therapy modification

Cefpodoxime: Histamine H2 Receptor Antagonists may decrease the absorption of Cefpodoxime. Separate oral doses by at least 2 hours. Monitor therapy

Cefuroxime: Histamine H2 Receptor Antagonists may decrease the absorption of Cefuroxime. Separate oral doses by at least 2 hours. Avoid combination

Chlormethiazole: Gastab may increase the serum concentration of Chlormethiazole. Monitor therapy

Chloroquine: Gastab may increase the serum concentration of Chloroquine. Monitor therapy

Cisapride: Gastab may increase the serum concentration of Cisapride. Management: Consider alternatives to Gastab. If this combination cannot be avoided, monitor for toxic effects of cisapride if Gastab is initiated/dose increased, or decreased efficacy if Gastab is discontinued/dose decreased. Consider therapy modification

Citalopram: Gastab may increase the serum concentration of Citalopram. Monitor therapy

CloZAPine: Gastab may increase the serum concentration of CloZAPine. Management: Consider use of an alternative H2 antagonist. Monitor for increased toxic effects of clozapine if Gastab is initiated/dose increased, or decreased effects if Gastab is discontinued/dose decreased. Consider therapy modification

Cysteamine (Systemic): Histamine H2 Receptor Antagonists may diminish the therapeutic effect of Cysteamine (Systemic). Monitor therapy

Dacomitinib: Histamine H2 Receptor Antagonists may decrease the serum concentration of Dacomitinib. Management: Administer dacomitinib at least 6 hours before or 10 hours after an histamine H2-receptor antagonist (H2RA). Consider therapy modification

Dalfampridine: Gastab may increase the serum concentration of Dalfampridine. Management: Recommendations differ significantly between international labelings in regards to the concomitant use of dalfampridine (referred to as fampridine in Canada) and Gastab. Consult appropriate product labeling. Monitor therapy

Dasatinib: Histamine H2 Receptor Antagonists may decrease the absorption of Dasatinib. Management: Antacids (taken 2 hours before or after dasatinib administration) can be used in place of H2-antagonists if some acid-reducing therapy is needed. Avoid combination

Delavirdine: Histamine H2 Receptor Antagonists may decrease the serum concentration of Delavirdine. Management: Chronic therapy with H2-antagonists should be avoided in patients who are being treated with delavirdine. The clinical significance of short-term H2-antagonist therapy with delavirdine is uncertain, but such therapy should be undertaken with caution. Avoid combination

Dexmethylphenidate: Histamine H2 Receptor Antagonists may increase the absorption of Dexmethylphenidate. Specifically, H2-antagonists may interfere with the normal release of drug from the extended-release capsules (Focalin XR brand), which could result in both increased absorption (early) and decreased delayed absorption. Monitor therapy

Dofetilide: Gastab may increase the serum concentration of Dofetilide. This is likely via inhibition of dofetilide renal tubular secretion (primarily) and inhibition of dofetilide metabolism. Avoid combination

Doxofylline: Gastab may increase the serum concentration of Doxofylline. Monitor therapy

EpiRUBicin: Gastab may increase the serum concentration of EpiRUBicin. Avoid combination

Erdafitinib: May increase the serum concentration of P-glycoprotein/ABCB1 Substrates. Monitor therapy

Erdafitinib: May increase the serum concentration of OCT2 Substrates. Monitor therapy

Erlotinib: Histamine H2 Receptor Antagonists may decrease the serum concentration of Erlotinib. Management: Avoid H2-antagonists in patients receiving erlotinib when possible. If concomitant treatment cannot be avoided, erlotinib should be dosed once daily, 10 hours after and at least 2 hours before H2-antagonist dosing. Consider therapy modification

Escitalopram: Gastab may increase the serum concentration of Escitalopram. Monitor therapy

Flibanserin: CYP3A4 Inhibitors (Weak) may increase the serum concentration of Flibanserin. Monitor therapy

Flunitrazepam: Gastab may increase the serum concentration of Flunitrazepam. Monitor therapy

Fluorouracil Products: Gastab may increase the serum concentration of Fluorouracil Products. Monitor therapy

FLUoxetine: Gastab may increase the serum concentration of FLUoxetine. Monitor therapy

Fosamprenavir: Histamine H2 Receptor Antagonists may decrease the serum concentration of Fosamprenavir. Gastab may also inhibit the metabolism of the active metabolite amprenavir, making its effects on fosamprenavir/amprenavir concentrations difficult to predict. Monitor therapy

Fosphenytoin-Phenytoin: Gastab may enhance the adverse/toxic effect of Fosphenytoin-Phenytoin. Gastab may increase the serum concentration of Fosphenytoin-Phenytoin. Management: Consider using an alternative H-antagonist to avoid this interaction. Monitor for toxic effects of hydantoin anticonvulsants if Gastab is initiated/dose increased. Consider therapy modification

Gefitinib: Histamine H2 Receptor Antagonists may decrease the serum concentration of Gefitinib. Management: Administer gefitinib at least 6 hours before or after administration of a histamine H2-antagonist, and closely monitor clinical response to gefitinib. Consider therapy modification

Indinavir: Histamine H2 Receptor Antagonists may decrease the serum concentration of Indinavir. Monitor therapy

Iron Preparations: Histamine H2 Receptor Antagonists may decrease the absorption of Iron Preparations. Exceptions: Ferric Carboxymaltose; Ferric Citrate; Ferric Derisomaltose; Ferric Gluconate; Ferric Hydroxide Polymaltose Complex; Ferric Pyrophosphate Citrate; Ferumoxytol; Iron Dextran Complex; Iron Sucrose. Monitor therapy

Itraconazole: Histamine H2 Receptor Antagonists may increase the serum concentration of Itraconazole. Histamine H2 Receptor Antagonists may decrease the serum concentration of Itraconazole. Management: Administer Sporanox brand itraconazole at least 2 hours before or 2 hours after administration of any histamine H2 receptor antagonists (H2RAs). Exposure to Tolsura brand itraconazole may be increased by H2RAs; consider itraconazole dose reduction. Consider therapy modification

Ketoconazole (Systemic): Histamine H2 Receptor Antagonists may decrease the serum concentration of Ketoconazole (Systemic). Management: Administer oral ketoconazole at least 2 hours prior to use of any H2-receptor antagonist. Monitor patients closely for signs of inadequate clinical response to ketoconazole. Consider therapy modification

Lasmiditan: May increase the serum concentration of P-glycoprotein/ABCB1 Substrates. Avoid combination

Ledipasvir: Histamine H2 Receptor Antagonists may decrease the serum concentration of Ledipasvir. Consider therapy modification

Lemborexant: CYP3A4 Inhibitors (Weak) may increase the serum concentration of Lemborexant. Management: The maximum recommended dosage of lemborexant is 5 mg, no more than once per night, when coadministered with weak CYP3A4 inhibitors. Consider therapy modification

Lomitapide: CYP3A4 Inhibitors (Weak) may increase the serum concentration of Lomitapide. Management: Patients on lomitapide 5 mg/day may continue that dose. Patients taking lomitapide 10 mg/day or more should decrease the lomitapide dose by half. The lomitapide dose may then be titrated up to a max adult dose of 30 mg/day. Consider therapy modification

Lumacaftor and Ivacaftor: May decrease the serum concentration of P-glycoprotein/ABCB1 Substrates. Lumacaftor and Ivacaftor may increase the serum concentration of P-glycoprotein/ABCB1 Substrates. Monitor therapy

Mebendazole: Gastab may increase the serum concentration of Mebendazole. Monitor therapy

Melatonin: Gastab may increase the serum concentration of Melatonin. Monitor therapy

Meperidine: Gastab may increase the serum concentration of Meperidine. Monitor therapy

Mesalamine: Histamine H2 Receptor Antagonists may diminish the therapeutic effect of Mesalamine. Histamine H2-Antagonist-mediated increases in gastrointestinal pH may cause the premature release of mesalamine from specific sustained-release mesalamine products. Management: Consider avoiding concurrent administration of high-dose histamine H2-receptor antagonists with sustained-release mesalamine products. Consider therapy modification

MetFORMIN: Gastab may increase the serum concentration of MetFORMIN. Management: Consider alternatives to Gastab in patients receiving metformin due to a potential for increased metformin concentrations and toxicity (including lactic acidosis). Consider therapy modification

Methylphenidate: Histamine H2 Receptor Antagonists may increase the absorption of Methylphenidate. Specifically, H2-antagonists may interfere with the normal release of drug from the extended-release capsules (Ritalin LA brand), which could result in both increased absorption (early) and decreased delayed absorption. Monitor therapy

Mirtazapine: Gastab may increase the serum concentration of Mirtazapine. Monitor therapy

Moclobemide: Gastab may decrease the metabolism of Moclobemide. Management: Consider using alternative agents to lower gastric pH in order to avoid this interaction. If combined, a moclobemide dose reduction of 50% is recommended and patients should be monitored for increased moclobemide effects/toxicities. Consider therapy modification

Multivitamins/Minerals (with ADEK, Folate, Iron): Histamine H2 Receptor Antagonists may decrease the serum concentration of Multivitamins/Minerals (with ADEK, Folate, Iron). Specifically, the absorption of iron may be impaired by H2-antagonists. Monitor therapy

Nelfinavir: Histamine H2 Receptor Antagonists may decrease the serum concentration of Nelfinavir. Concentrations of the active M8 metabolite may also be reduced. Monitor therapy

Neratinib: Histamine H2 Receptor Antagonists may decrease the serum concentration of Neratinib. Specifically, histamine H2 receptor antagonists may reduce neratinib absorption. Management: Administer neratinib at least 2 hours before or 10 hours after administration of a histamine H2 receptor antagonist to minimize the impact of this interaction. Consider therapy modification

Nicotine: Gastab may increase the serum concentration of Nicotine. Monitor therapy

Nilotinib: Histamine H2 Receptor Antagonists may decrease the serum concentration of Nilotinib. Management: The nilotinib dose should be given 10 hours after or 2 hours before the H2 receptor antagonist in order to minimize the risk of a significant interaction. Consider therapy modification

Nitisinone: May increase the serum concentration of OAT1/3 Substrates. Monitor therapy

PARoxetine: Gastab may increase the serum concentration of PARoxetine. Monitor therapy

PAZOPanib: Histamine H2 Receptor Antagonists may decrease the serum concentration of PAZOPanib. Management: Avoid the use of histamine H2-antagonists in combination with pazopanib. Strategies to minimize the expected interaction between these agents (eg, dose separation) have not been investigated. Avoid combination

Pentoxifylline: Gastab may increase the serum concentration of Pentoxifylline. Monitor therapy

Perhexiline: CYP2D6 Inhibitors (Weak) may increase the serum concentration of Perhexiline. Monitor therapy

Pexidartinib: Histamine H2 Receptor Antagonists may decrease the serum concentration of Pexidartinib. Management: Administer pexidartinib 2 hours before or 10 hours after histamine H2 receptor antagonists. Consider therapy modification

P-glycoprotein/ABCB1 Inducers: May decrease the serum concentration of P-glycoprotein/ABCB1 Substrates. P-glycoprotein inducers may also further limit the distribution of p-glycoprotein substrates to specific cells/tissues/organs where p-glycoprotein is present in large amounts (e.g., brain, T-lymphocytes, testes, etc.). Monitor therapy

P-glycoprotein/ABCB1 Inhibitors: May increase the serum concentration of P-glycoprotein/ABCB1 Substrates. P-glycoprotein inhibitors may also enhance the distribution of p-glycoprotein substrates to specific cells/tissues/organs where p-glycoprotein is present in large amounts (e.g., brain, T-lymphocytes, testes, etc.). Monitor therapy

Pilsicainide: Gastab may increase the serum concentration of Pilsicainide. Monitor therapy

Pimozide: CYP3A4 Inhibitors (Weak) may increase the serum concentration of Pimozide. Avoid combination

Posaconazole: Histamine H2 Receptor Antagonists may decrease the serum concentration of Posaconazole. Management: Avoid concurrent use of oral suspension with H2-antagonists whenever possible. Monitor patients closely for decreased antifungal effects if this combination is used. Delayed-release posaconazole tablets may be less likely to interact. Consider therapy modification

Pramipexole: Gastab may increase the serum concentration of Pramipexole. Monitor therapy

Praziquantel: Gastab may increase the serum concentration of Praziquantel. Monitor therapy

Pretomanid: May increase the serum concentration of OAT1/3 Substrates. Monitor therapy

Procainamide: Gastab may increase the serum concentration of Procainamide. Management: Consider an alternative H2-receptor antagonist in patients taking procainamide. If combined, monitor for increased therapeutic effects/toxicity of procainamide. Consider therapy modification

Propafenone: Gastab may increase the serum concentration of Propafenone. Monitor therapy

QuiNIDine: Gastab may increase the serum concentration of QuiNIDine. Management: Consider alternatives to Gastab. If the combination cannot be avoided, monitor for increased quinidine concentrations/toxicity with Gastab initiation/dose increase, or decreased concentrations/effects with Gastab discontinuation/dose decrease. Consider therapy modification

QuiNINE: Gastab may increase the serum concentration of QuiNINE. Management: Consider using an alternative H2-receptor antagonist (eg, ranitidine) instead of Gastab due to a lower interaction risk. If combined, monitor patients closely for signs and symptoms of quinine toxicity. Consider therapy modification

Ranolazine: May increase the serum concentration of P-glycoprotein/ABCB1 Substrates. Monitor therapy

Rilpivirine: Histamine H2 Receptor Antagonists may decrease the serum concentration of Rilpivirine. Management: Administer histamine H2 receptor antagonists at least 12 hours before or 4 hours after rilpivirine. Consider therapy modification

Risedronate: Histamine H2 Receptor Antagonists may increase the serum concentration of Risedronate. This applies specifically to delayed-release risedronate. Avoid combination

Roflumilast: Gastab may increase serum concentrations of the active metabolite(s) of Roflumilast. Gastab may increase the serum concentration of Roflumilast. Monitor therapy

Saquinavir: Histamine H2 Receptor Antagonists may increase the serum concentration of Saquinavir. Monitor therapy

Secretin: Histamine H2 Receptor Antagonists may diminish the diagnostic effect of Secretin. Specifically, use of H2-Antagonists may cause a hyperresponse in gastrin secretion in response to secretin stimulation testing, falsely suggesting gastrinoma. Management: Avoid concomitant use of histamine H2-antagonists (H2RAs) and secretin. Discontinue H2RAs at least 2 days prior to secretin administration. Consider therapy modification

Sulfonylureas: Gastab may increase the serum concentration of Sulfonylureas. Monitor therapy

Tafenoquine: May increase the serum concentration of OCT2 Substrates. Management: Avoid use of OCT2 substrates with tafenoquine, and if the combination cannot be avoided, monitor closely for evidence of toxicity of the OCT2 substrate and consider a reduced dose of the OCT2 substrate according to that substrate's labeling. Consider therapy modification

Tamsulosin: Gastab may increase the serum concentration of Tamsulosin. Monitor therapy

Teriflunomide: May increase the serum concentration of OAT1/3 Substrates. Monitor therapy

Theophylline Derivatives: CYP1A2 Inhibitors (Weak) may increase the serum concentration of Theophylline Derivatives. Exceptions: Dyphylline. Monitor therapy

TiZANidine: CYP1A2 Inhibitors (Weak) may increase the serum concentration of TiZANidine. Management: Avoid these combinations when possible. If combined use is necessary, initiate tizanidine at an adult dose of 2 mg and increase in 2 to 4 mg increments based on patient response. Monitor for increased effects of tizanidine, including adverse reactions. Consider therapy modification

Tolvaptan: May increase the serum concentration of OAT1/3 Substrates. Management: Avoid concomitant use of OAT1/3 substrates in patients receiving the Jynarque brand of tolvaptan. Concentrations and effects of the OAT1/3 substrate would be expected to increase with combined use. Consider therapy modification

Triazolam: CYP3A4 Inhibitors (Weak) may increase the serum concentration of Triazolam. Management: Consider triazolam dose reduction in patients receiving concomitant weak CYP3A4 inhibitors. Consider therapy modification

Tricyclic Antidepressants: Gastab may decrease the metabolism of Tricyclic Antidepressants. Monitor therapy

Ubrogepant: CYP3A4 Inhibitors (Weak) may increase the serum concentration of Ubrogepant. Management: In patients taking weak CYP3A4 inhibitors, the initial and second dose (if needed) of ubrogepant should be limited to 50 mg. Consider therapy modification

Urapidil: Gastab may enhance the hypotensive effect of Urapidil. Monitor therapy

Varenicline: Histamine H2 Receptor Antagonists may increase the serum concentration of Varenicline. Management: Monitor for increased varenicline adverse effects with concomitant use of Gastab or other H2-antagonists, particularly in patients with severe renal impairment. International product labeling recommendations vary. Consult appropriate labeling. Monitor therapy

Velpatasvir: Histamine H2 Receptor Antagonists may decrease the serum concentration of Velpatasvir. Monitor therapy

Vitamin K Antagonists (eg, warfarin): Gastab may enhance the anticoagulant effect of Vitamin K Antagonists. Management: Avoid coadministration of Gastab and vitamin K antagonists. If unavoidable, monitor for increased effects of vitamin K antagonists when Gastab is initiated/dose increased, or decreased effects if Gastab is discontinued/dose decreased. Consider therapy modification

Zaleplon: Gastab may increase the serum concentration of Zaleplon. Management: The initial dose of zaleplon should be limited to 5 mg in patients taking Gastab. Monitor patients for increased zaleplon effects/toxicities (ie, sedation, CNS depression) when these agents are combined. Consider therapy modification

ZOLMitriptan: Gastab may increase the serum concentration of ZOLMitriptan. Management: Limit the maximum single dose of zolmitriptan to 2.5 mg, not to exceed 5 mg in 24 hours, when coadministered with Gastab. Consider therapy modification

See also:
What are the possible side effects of Gastab?

Adverse effects reported in patients taking Gastab are described below by body system. Incidence figures of 1 in 100 and greater are generally derived from controlled clinical studies.

Gastrointestinal

Diarrhea (usually mild) has been reported in approximately 1 in 100 patients.

CNS

Headaches, ranging from mild to severe, have been reported in 3.5% of 924 patients taking 1600 mg/kg, 2.1% of 2,225 patients taking 800 mg/day and 2.3% of 1,897 patients taking placebo. Dizziness and somnolence (usually mild) have been reported in approximately 1 in 100 patients on either 1600 mg/day or 800 mg/day.

Reversible confusional states, e.g., mental confusion, agitation, psychosis, depression, anxiety, hallucinations, disorientation, have been reported predominately but not exclusively, in severely ill patients. They have usually developed within 2–3 days of initiation of Gastab therapy and have cleared within 3–4 days of discontinuation of the drug.

Endocrine

Gynecomastia has been reported in patients treated for one month or longer. In patients being treated for pathological hypersecretory states, this occurred in about 4% of cases while in all others the incidence was 0.3% to 1% in various studies. No evidence of induced endocrine dysfunction was found, and the condition remained unchanged or returned toward normal with continuing Gastab treatment.

Reversible impotence has been reported in patients with pathological hypersecretory disorders, e.g., Zollinger-Ellison Syndrome, receiving Gastab particularly in high doses, for at least 12 months (range 12–79 months, mean 38 months). However, in large-scale surveillance studies at regular dosage, the incidence has not exceeded that commonly reported in the general population.

Hematologic

Decreased white blood cell counts in Gastab-treated patients (approximately 1 per 100,000 patients), including agranulocytosis (approximately 3 per million patients), have been reported, including a few reports of recurrence on rechallenge. Most of these reports were in patients who had serious concomitant illnesses and received drugs and/or treatment known to produce neutropenia. Thrombocytopenia (approximately 3 per million patients) and, very rarely, cases of pancytopenia or aplastic anemia have also been reported. As with some other H2-receptor antagonists, there have been extremely rare reports of immune hemolytic anemia.

Hepatobiliary

Dose-related increases in serum transaminase have been reported. In most cases they did not progress with continued therapy and returned to normal at the end of therapy. There have been rare reports of cholestatic or mixed cholestatic-hepatocellular effects. These were usually reversible. Because of the predominance of cholestatic features, severe parenchymal injury is considered highly unlikely. However, as in the occasional liver injury with other H2-receptor antagonists, in exceedingly rare circumstances fatal outcomes have been reported.

There has been reported a single case of biopsy-proven periportal hepatic fibrosis in a patient receiving Gastab.

Rare cases of pancreatitis, which cleared on withdrawal of the drug, have been reported.

Hypersensitivity

Rare cases of fever and allergic reactions including anaphylaxis and hypersensitivity vasculitis, which cleared on withdrawal of the drug have been reported.

Renal

Small, possibly dose-related increases in plasma creatinine, presumably due to competition for renal tubular secretion, are not uncommon and do not signify deteriorating renal function. Rare cases of interstitial nephritis and urinary retention, which cleared on withdrawal of the drug, have been reported.

Cardiovascular

Rare cases of bradycardia, tachycardia and A-V heart block have been reported with H2-receptor antagonists.

Musculoskeletal

There have been rare reports of reversible arthralgia and myalgia; exacerbation of joint symptoms in patients with preexisting arthritis has also been reported. Such symptoms have usually been alleviated by a reduction in Gastab dosage. Rare cases of polymyositis have been reported, but no causal relationship has been established.

Integumental

Mild rash and, very rarely, cases of severe generalized skin reactions including Stevens-Johnson syndrome, epidermal necrolysis, erythema multiforme, exfoliative dermatitis and generalized exfoliative erythroderma have been reported with H2-receptor antagonists. Reversible alopecia has been reported very rarely.

Immune Function

There have been extremely rare reports of strongyloidiasis hyperinfection in immunocompromised patients.